1. Field of the Invention
The present invention relates to the removal of contaminants entrained in a gas stream.
2. Discussion of the Background
Many different types of manufacturing plants produce exhaust gases that have pollutants, such as polycyclic aromatic hydrocarbons (PAH), entrained in the stream of exhaust gases. Streams of exhaust gases from manufacturing plants can contain hydrocarbons such as oils, petroleum fractions, benzenes, and solvents. There are numerous examples of exhaust gas streams from manufacturing plants that contain PAH, such as spent protective atmosphere gases, coke oven gas, and various refinery waste streams.
The presence of such pollutants in exhaust gas streams raises several issues for consideration when determining how to deal with or utilize the exhaust gas stream. Such pollutants raise environmental concerns in situations where venting of the exhaust gases into the surrounding atmosphere is considered. At many locations, the emission of pollutants into the atmosphere is regulated, and extra emissions may be strictly prohibited, requiring the addition of expensive post-treatment equipment, such as thermal or catalytic oxidizers. Additionally, in some circumstances, the exhaust gases might be considered for recovery and recycling, in order to reuse the exhaust gases as fuel in the manufacturing plant or for other purposes. However, due to the pollutants in the exhaust gases, the fuel value of the recovered exhaust gases may be low. Therefore, it may be necessary to remove the pollutants from the exhaust gases before discharging the exhaust gases into the atmosphere, or before reusing the exhaust gases as a fuel or process feedstock.
Pollutants that are entrained in the exhaust stream in the liquid phase in the form of large droplets can be separated from the exhaust stream by using baffle separators, cyclones, demisters, electric separators, or similar devices. These devices are capable of separating particles as small as 0.5 μm. However, it has proven to be difficult to separate out pollutants that are entrained in the exhaust stream in the gaseous phase or even in ultra fine droplets. Activated carbon can be used to remove gaseous pollutants from gas streams. However, when dealing with large quantities of exhaust gases that can contain large amounts of pollutants (e.g. exhaust gases of a steel annealing plant), the use of filters that contain active carbon can be unfeasible. Firstly, filters that contain active carbon are extremely expensive. Additionally, several kilograms of pollutants can clog even a large carbon filter rendering it unsuitable for use, so that frequent change of filters becomes necessary and the removal of spent filter content becomes a problem.